Measuring Voltage Across Capacitor

MIshka1177

13th May 2008 - 04:16 PM

Hi guys, thought I would need a collective brain here Hope some would find this interesting and not stupid

So, the problem: Let's say that I have a circuit that acts kind of like a dynamic battery, generating current and thus potential (mV range). It is completely isolated and only shares ground with my measuring circuit. Now, the only way to connect to this theoretical circuit is across a capacitor with, let's say, a known value. The problem is, this capacitor cannot be shorted. So basically I need to measure "sub-threshold" potential fluctuations on one side of that capacitor.

I am thinking along the lines of a condenser microphone concept, whereby I would build up a bias by applying the charge on the measuring side of that capacitor, and going from there...I can assume I have an extremely high input resistance amplifier to measure whatever.

Any ideas?

Ron

13th May 2008 - 06:41 PM

Hi Mishka,
First off, you need to know the frequency of your supply. then you can calculate a range of capacitors that will give you the reactance (or ac resistance ) that you would like. Xc = [1/(2pi x f x c)] where Xc is capacitive reactance in ohms.
I'm not really sure of what your trying to do, but this should help a little.
Peace,
Ron

MIshka1177

13th May 2008 - 07:51 PM

QUOTE (Ron+May 13 2008, 06:41 PM)

Hi Ron, thanks a ton for the reply and some ideas! Too tired to really think right now (in Sweden), so will revisit tomorrow.

I am very rusty on this side of things (have been doing something entirely different for years), so bear with me...

I tried my best to explain the jist of the problem by using an equivalent model (the real thing would just take too long to explain), but I think I thought of another, better one. Let's just say that I have a long antenna (generates low current and voltage signals, the events are in the in mV and 1-2kHz range), connected to my amplifier through a fixed-value (which I am stuck with) capacitor. I need to sample those events somehow. Let's assume I can generate any sort and amplitude signal on the amp side.

Ron

13th May 2008 - 08:17 PM

Hi Mishka,
Try a Piezoelectric speaker. Google some and look at the specs.
Otherwise, try a headphone speaker (and put an 8 ohm load across it if you need to).
Later,
Ron

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